Moto Guzzi CARC shaft drive

CARC2

By Kevin Ash


Pictures: Moto Guzzi Press




Cardano Reattivo Compatto isn’t a phrase you hear tripping off the tongues of many motorcyclists, nor even its acronym CARC, inscribed on the shaft drive housings of the current range of Moto Guzzis. It’s interesting though, if you’d like to know why Guzzis no longer pogo up and down like Zebedee at a Clash gig when you open and close the throttle.

CARCIt doesn't look like chain drive but it behaves like itI cover in another feature (to be published here later) why the back ends of simple shaft drive bikes pitch up when power is applied, so just a brief summary now: when the throttle is opened the rear wheel is driving the whole bike forwards by pushing horizontally against the swingarm with its spindle. Because the swingarm is angled downwards from its pivot point, this also has the effect of extending the rear suspension – the rear wheel is trying to force its way underneath the bike, jacking up the back as it does so. Close the throttle and the back end drops down again, upsetting the handling in the process. Chain drive bikes do this much less because the chain is pulling the top of the rear sprocket forwards, which is trying to compress the suspension and in turn countering the jacking-up effect.

Guzzi’s CARC, translates as Compact Reactive Drive (a cardan shaft – the ‘cardano’ in Italian – is any shaft used to transmit power which has one or more universal joints in it). What this does is separate the forces of the power train from the swingarm itself. The swingarm on the right hand side is a large diameter hollow tube with no joints or pivots, so it’s very rigid and ideal for handling integrity. The drive shaft, which has two universal joints, is carried inside it, but it’s free to float and has no direct affect on the swingarm.

CARC2All modern big capacity Guzzis feature the CARC systemThe bevel gear assembly is also not attached directly to the swingarm. Instead, it’s kept in place by a short arm which sticks out of the top of the final drive housing, which is connected to a long tie rod running above the swingarm and anchored at the other end directly to the bike’s frame. It’s the geometry of this arrangement which is the important part. As power is applied from the engine, through the clutch and gearbox to the drive shaft, the small ‘planet’ bevel gear tries to move around the large ‘sun’ gear attached to the wheel. But this planet gear is mounted on an assembly rigidly attached to the short vertical arm, which holds it all in place by pulling on the long tie rod, and it’s the tension in this part of the system which pulls upwards on the suspension, counteracting the down force of the wheel forcing its way under the bike and extending the suspension.

The end result is behaviour which is effectively the same as a chain drive bike. The familiar pro’s and con’s remain the same: more unsprung mass so the suspension movement is more sluggish, but maintenance is low and it stays cleaner. Otherwise, you’d hardly tell the difference between CARC shaft and chain.

* Moto Guzzi Stelvio NTX test


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